The overall goal of this research is to understand the regulation of bone and cartilage cell growth and specialization. Ongoing in vitro studies using osteoblast-like cells and chondrocytes isolated from the calvaria of rat fetuses as experimental models have yielded two observations which have generated our present specific aims. First, in testing the hypothesis that cells of the monocyte-macrophage series regulate bone formation, we have found that macrophages elaborate a bone cell and chondrocyte mitogenic protein (macrophage-derived growth factor, MDGF). Second, in exploring possible mechanisms for the activation of bone remodeling, we found that certain agents, including prostaglandins of the E series, cause acute alterations in bone cell shape and cell-cell contact which may facilitate activation and intercellular communication. The specific aims of this proposal extend each of these lines of inquiry. We will: a) undertake the purification to homogeneity and characterization of MDGF from incubation media conditioned by a macrophage-like cell line (monoclonal antibodies will be prepared to MDGF, in order to aid in its purification, its detection in cells and in biological fluids, and in understanding its mode of action); b) study specific macrophage subtypes and macrophage-like cell lines engaged in MDGF synthesis; c) explore the regulation of MDGF synthesis by humoral agents (such as prostaglandins, glucocorticoids, estrogens, 1 Alpha,25(OH)2 vitamin D3) and by non-humoral manipulations (exposure to bone tissue and its components, and lymphokines); and d) examine the nature of bone cell and chondrocyte responses to MDGF and the effect of MDGF on bone resorption. We will probe the mechanisms of and relationships among bone cell shape change, collagenase production and gap junction formation in response to osteotropic hormones. These studies are expected to enhance our knowledge of bone formation, remodeling and repair, so crucial to understanding skeletal disease, and could lead to novel strategies for disease prevention and treatment.